Overhead projectors of the transmissive type include a base unit which encloses a light source and lenses for suitably focussing light from the source and a transparent stage upon which may be positioned a transparency or other film having indicia which is to be projected on a distant screen. Above the base is disposed a projection head having lenses and a mirror which function to gather light from the projector base and redirect it to the screen. The projection head is supported by a post extending upwardly from the base. It is necessary that the distance between the projection head and the base be adjustable so that the image projected on the screen may be brought into focus. This is conventionally accomplished by providing a rack gear on the post and a pinion gear attached to an arm which supports the projection head. The pinion gear is turned by means of a handle to move the arm and the attached projection head in the desired direction until the projected image is in focus.
In order to produce and maintain a satisfactory image at the screen it is necessary that the arm supporting the projection head be firmly positioned relative to the base and free of movement in any direction other than perpendicular to the plane of the projector stage. Furthermore, it is necessary that the connection between the projector focus arm and the post be able to compensate for wear between the arm and the post so that alignment and stable support is maintained throughout the life of the projector. On the other hand, the focus arm must be free to move smoothly and without undue force from one vertical position to another.
In the past it has proven difficult to simultaneously achieve all the desirable aspects of the connection between the projector head focus arm and the post extending from the projector base.